text:
When, if any case, can it be considered justifiable to reject a takeoff after V1 speed, if the aircraft is incapable of taking off?
Not a pilot, but I've been a passenger before and I'm writing a sci-fi book trying to use realistic aviation terms (so ignore my use of slang and acronyms here). I understand that V1 is when you have to keep going and there isn't enough room on the runway (realistically speaking) to reject the takeoff without suffering a runway excursion.
However, it would also seem that under those same grounds, there are rare cases in which the PIC may have to abort because the aircraft literally cannot take off (i.e. stuck control surface = plane can't take off at all). In these exceedingly rare cases where the only possible option is to stop - even if doing so would overrun the runway, here are my questions:

How brief would the "window of opportunity" be to reject takeoff at V1 be? That is, would the non-PIC calling out "V1" and less than half a second later the PIC orders an abort (due to being unable to take off) make a difference as compared to a rejected takeoff five or ten seconds post-V1? It would seem to me that the chances of actually pulling off a successful rejection and not overrunning the runway would increase with less time elapsed after the V1 callout.

Besides the control surface problem, are there any other circumstances you envision where the aircraft cannot take off that mean you have no choice but to reject takeoff after V1? (And, on a more morbid note, if there was a runway incursion involving a person at V1, my book has the ship hit them and take off regardless, so I wonder if that's the case for the plane too - if something happens at V1, you have to fly the plane, regardless of what's in the way). Or to be more precise: what are the grounds, in your opinion, that justify rejecting takeoff at V1?

Even assuming the rejection was successful enough that either the aircraft doesn't overrun the runway or the aircraft does overrun the runway but no one dies, what kind of consequences can the PIC/non-PIC expect?

Related: What if you have a double-engine failure after V1 but before VR?

@AndreasRejbrand: V1 is calculated assuming thrust reversers are inoperable.

Could you re-phrase that, please?
Surely in every case, it's necessary to reject a takeoff if the aircraft is incapable of taking off?
Isn't that half the purpose of V1?

@RobbieGoodwin V1 is meaningful for a case in which the pilot might go flying but also might try to stop. If your dry runway V1 is capped at rotation speed, you're probably capable of continuing the takeoff even if an engine fails somewhat before V1... because if the runway were icy & slick, your lesser ability to stop might mean you'd commit to flying sooner than VR. So in some range of speeds (dry rwy), you could do either; V1 is where you commit to go fly. Also, while single engine aircraft can have a "max refusal speed", they have no V1. There, 1 engine out = "incapable of flight"!

Uh… really, Ralph? I thought everyone knew V1 was when you could and V2 when you must take off.

@RalphJ Yes, exactly.

If the aircraft is truly incapable of taking off -- the locked controls case, let's say -- then the only choice involved is along the lines of, go off the end of the runway really really fast (you kept accelerating once past takeoff speed), or go off the end of the runway not so fast, with max braking, max reverse, full spoilers, etc.  So if both engines just failed, it matters not how fast you are in relation to V1, you'd better start stopping, because you can't go flying -- at least, not for very long, and that certainly wouldn't end well either!
In the world of high-performance jets, the V1 speed is the speed at which you have determined you will commit to taking off.  That's it.  Before that speed, if I get a serious problem, I'll stop; above that speed, even with a serious problem (typically, along the lines of a fire or an engine failure), I'll continue the takeoff and deal with the problem in the air.  And the performance data supports continuing the takeoff from that point even with one engine out.
So the statement -- which you'll see many, many places -- that "above V1 you can't abort & stop on the runway" isn't always accurate.  If I have a powerful & light aircraft that needs 3,000' of runway to accelerate to takeoff speed, and another 3,000' to come to a stop if an abort is initiated at takeoff speed, and I'm taking off from a 12,000' long runway, then the speed that I'll use for the "V1" callout will be my takeoff (i.e. "rotation") speed, VR.  But, could I successfully abort from a speed faster than that in this case?  Absolutely.
Consider the case when my jet has a light load of passengers + fuel, and the identical aircraft taking off behind me has a heavy load.  His VR speed, when he rotates up and starts to go flying, will be higher than mine because he has a heavier aircraft.  So maybe he needs 4,000' to accelerate to his takeoff speed, and 4,500' feet to stop from that speed.  So his VR is above mine, and his V1 matches his VR, so his V1 is higher than mine... well, if he can stop on the runway having initiated an abort at the higher speed, in my lighter aircraft I certainly could also initiate an abort at that higher speed and stop successfully.
We cap our V1 speed at VR, because it doesn't make sense, in a planning & performance standpoint, to consider rejecting the takeoff after you've started to rotate & go fly.  But at any time up until that V1 speed, you aren't committed to flying yet... you can still abort.
As for timing, how quickly the Pilot Flying would have to initiate the abort after event (let's say the "bang" of an engine seizing -- a really violent engine failure), that's defined by the manufacturer and incorporated into the performance data.  They test how long, in given conditions, it takes from the time the pilot starts the abort procedure, until the aircraft is stopped.
Then they build in assumptions (which they state in the performance data) about reaction time and so on.  So if the test pilots, knowing what's coming, reach XXX knots & shut down an engine, they'll delay some time (perhaps 1 second) to say "Abort!", reflecting that everybody besides the test pilots has to process what just happened & will take a non-zero timespan to determine than an engine failure occurred.
Then, having stated "abort," they may delay one more second, and initiate the abort procedure -- perhaps one engine in idle (simulating, failed), one engine in reverse, full braking on the wheels, full spoilers.  And when the aircraft stops, that distance is the data point.
Then they repeat that test at enough different conditions (gross weight, temperature, etc) so that they can confidently draw up performance charts so that I can tell, on "this" day with "these" conditions that I have, what is my accelerate-stop distance and speed -- what will it take for me to go from stopped, to takeoff speed, to stopped again, using all the given assumptions?  If my VR speed is higher than this speed, then my V1 will be below my VR (and at, or perhaps slightly below, this accelerate-stop speed determined from the charts & the available runway).  Or, if I can accelerate to a speed higher than my takeoff speed & still stop within the available runway, then my V1 probably matches my VR.
So to your (a) question above, yes, the odds of "successfully" aborting above V1 depend greatly on how far above V1 the pilot initiates the abort.  If it's a half-second beyond, maybe the jet stops in the overrun, with no or minor damage, while 5 seconds beyond, the jet is through the overrun, the approach lights, the boundary fence, and into the highway just outside the airport.  Or, maybe this was the day you only needed 6,000' to accelerate & stop again but you had 12,000', so despite the late abort everything turned out okay.  Or maybe the runway has EMAS, and that ended up protecting the approach lights, fence, and highway.
As for your question (b), this is where people get creative, and sometimes a little bit aircraft-specific.  A total electrical failure on the runway, at night with an overcast you'd be climbing into might be such a scenario.  In the T-38, there was a specific combination of generator failure with an additional electrical failure that left you with a pretty ugly set of instrument outages, and it was a discussion topic of, do you take the jet into IMC in this condition, or do you abort late & trust the barrier to stop you.  Maybe a massive armada of aircraft suddenly appearing at 200' with the certainty that going flying means a midair with one or more of them would qualify (like I said, getting creative here).
But for the most part, the aircraft designers have put lots of work into designing out as many of those sorts of scenarios as possible, so that you can safely go flying when an engine fails at or above V1.
For question (c), in an airline or military world at least, there would be an investigation into everything that happened, and did the PIC make an appropriate decision at the time, knowing what he knew and given the guidance that he had.  If he aborted many seconds beyond V1 because he got a warning light on the cabin pressurization panel, the investigation will probably determine that he screwed up pretty seriously.  Or, if he put the airplane into the EMAS rather than taking the total electrical failure airborne on the dark & stormy night, they could conclude that he made a really good decision.
Depending on how punitive the employer is and how badly he messed up, the pilot might be fired, or might get some (punitive) time off & some retraining, or there might simply be a debrief and some "lessons learned" for everyone, and he keeps on flying.  In the case of a sufficiently egregious screw-up and an NTSB investigation, the FAA might pull his license so he's done flying altogether.

How about the OP's proposed scenario, of control surfaces not working for some reason?  Like, you pass V1, get to VR, pull on the stick, but the nose doesn't come up (at all, or not enough).  You could keep gaining speed to see if that helps, but if it doesn't, you've made things much worse.  So I'd guess if you knew you had enough runway to slow most of the way down, and something safe-ish to go off into, you'd abort at the cost of some damage to the gear and the lawn and maybe lights and/or ILS antennas

But I guess this is why pre-flight checklists involve verifying flight-control movements!  So your elevator control cables snap?  In an action novel or movie, perhaps the heroes jumped into a plane with combat damage and didn't have time to run the checklists because baddies were hot on their trail.  (Or maybe these are the bad guys attempting escape.  Maybe they're also lax about flight safety procedures even when not being pursued.)

@PeterCordes That actually happened before: Texas MD-87 Crash Update: Jammed Elevators!. The MD-80 has tab-controlled elevators, so you cannot easily check these pre-flight.

Extreme vibration is reason to cancel past V1- the plane feels like it either going to rip itself apart or one or more engines are going explode- disintegrating engines are not guaranteed to be contained.

I read a case awhile back of a pilot discovering on lifting the nose wheel off the runway (between V1 and VR) he discovered rudder control left something to be desired. So he threw his training out the window so to speak, commanded elevators full down and maximum brakes, and overran into the runway lights. Investigation found hydraulic line rupture; that plane was unflyable so it's a good thing he aborted the takeoff.

Interflug 102 is another example, with the pilot flying realizing after V1 that the elevators were locked. It didn't end well, partly because the flight engineer immediately shut down all four engines, preventing the use of reverse thrust.

Think of V1 as a "bad things will likely happen if you're going faster than this when you abort" speed.  While not mentioned in other comments, this may often be a function of braking ability.  The usefulness of remaining runway will be greatly diminished if the brakes melt, but if that's going to happen, every extra knot of speed one gains before aborting takeoff will increase by more than a knot the speed with which one will leave the runway.  Since kinetic energy is proportional to the square of speed, and badness tends to scale with kinetic energy, every bit of speed reduction helps.


text:
How to detect orientation with accelerometer while vehicle has variable acceleration?
According to my research I saw that, in order to detect the angular orientation AHRS uses gyro and accelerometer. Only gyro does not work well because of integration drift. Only accelerometer also does not work because of vibrations and vehicle accelerations. That's why sensor fusion algorithms like Kalman filter are used to have a better guess.
What if we have a fighter jet that operates at excessive acceleration? In this case is it possible to extract gravitation vector from the accelerometer data? If we can't, that it means that at that operation we only rely on gyro which will drift after a while.
What kind of algorithms are used in these vehicles to fix the issue?

Same algorithms, better sensors :)

Noting first, that everything, including acceleration and angular orientation, is relative, not absolute, all these algorithms require establishing a baseline "acceleration" and "orientation" to compare with. Generally, the system has to first be "aligned" while at rest, (or injected with alignment data from an external source, for it to work properly.

Related: How can aircraft determine true north for inertial navigation systems?

You are right, an AHRS does have difficulties differentiating centrifugal accelerations from acceleration due to gravity. That is why in pretty much all higher-performance aircrafts utilize so called Inertial Navigation Systems coupled with GPS receivers. The advantage of these systems is that you now can measure/estimate the velocity of your aircraft. This does not sound like much, but it is! Because with the addition of this measurement/estimation you can calculate the centrifugal accelerations (which is a function of rotational rate and velocity) which then allows you to deconstruct your measurement into inertial and gravitational components. With the help of this, your algorithm is then able to compute your navigation state (rotational and translational) completely.
I write measurement/estimation, because in pretty much all applications I am aware of, a Kalman filter is used (as you correctly observed) to combine all available sensor data.
Some side note on AHRS units:

Typically, a modern AHRS unit will use a combination of Gyrometers, Accelerometers and Magnetometers, simply because by combining these sensors you can alleviate the weaknesses of each of the sensors.
Higher grade sensor do not make the problems dissapear, they are just weaker. For example drift is present in every single gyro, however with the most expensive FoGs, drift is very very low. But you still need to handle it (for example with the help of other Sensors and a Kalman filter), because even though it is low, it will add up given enough time.
Kalman filters pretty much always try to eliminate the gyro drift by explicitly estimating it. They even go so far to heat the gyroscope to some constant temperature, just so that the gyro drift remains constant (which is of course easier to estimate)

I find this website of an AHRS manufacturer to provide a good read. (I am not affiliated, before anyone asks...)

And, related: https://aviation.stackexchange.com/questions/86279/why-would-an-artificial-horizons-roll-axis-be-different-on-ground-and-in-flight

I looked closely at the link (last sentence of answer) was surprised to see no reference to GPS (GNSS) in the basic explanation of the AHRS.  Based on some previous research, I thought that that usually played a key role--

@quietflyer No, in my experience if a GPS-unit is added such a system would normally be advertised as an INS (although that might differ from the classical definition of an INS depending on how you view things). Looking at the same website, you might find this link helpful: https://www.vectornav.com/resources/inertial-navigation-primer/theory-of-operation/theory-gpsins. The AHRS can be useful in GPS-denied environment. Otherwise you would of course go for an INS with GPS, as it makes things a lot easier...

Btw I'm certain that the description for one particular, add-on system that I read about 5 years ago did describe how GPS data was used to help keep a AHRS system calibrated-- without making any reference to "INS"-- but perhaps this wasn't standard terminology-- I'm not that deeply familiar with the literature on the subject--

It's fairly simple:

If you integrate acceleration over time, you get velocity.
If you maintain substantial acceleration, you get ever-increasing velocity.
Aircraft have a maximum velocity. It cannot increase forever.

Conclusion: Aircraft cannot maintain substantial accelerations for extended periods of time. At some point, the aircraft must stop accelerating or turn around and accelerate in the other direction. If you average acceleration over a long enough period of time, it must vanish. What remains is gravity.
Example calculation:

Your aircraft has a mechanical attitude indicator. The time constant of the erection mechanism is 5 minutes. The maximum speed of your aircraft is Mach 0.9, around 300m/s.
What is the maximum turning error?

The maximum change in velocity is 600m/s (full speed in one direction to full speed in opposite direction) and we have to average that over 5 minutes. Even if you perform multiple turns during those 5 minutes, the change in velocity from the beginning of the first turn to the end of the last is never more than 600m/s. Whether you fly these turns at 2g, 10g, or 100g is utterly irrelevant, the change in velocity is still only 600m/s. The calculation is:
$$a_{max} = \frac{2\cdot v_{max}}{T} = \frac{600\frac{m}{s}}{300s} = 2\frac{m}{s^2} = 0.2g $$
The maximum acceleration that doesn't vanish is 0.2g, which translates to an angular error of:
$$ \arcsin\left(\frac{0.2g}{1.0g}\right) = 11.5°$$
A turning error of 11.5° is still substantial, but at least you know which way is up (and the error goes away if you fly straight for 5 minutes). You can reduce the turning error by using a weaker erection mechanism, but even with 10 or 15 minutes time constant the turning error is still 4-6° in the worst case. Larger time constants are problematic because earth turns with 15°/h, so during your 10 minutes of averaging the real gravity vector turned by 2.5°. The choice of time constant is a tradeoff between different kinds of errors. If you use cheap gyros with a drift rate of more than 15°/h, you have to consider that rate for making tradeoffs rather than the rotation of earth.

So the essence of this answer is that by simply making the time constant of the correction algorithm sufficiently large, it works just fine to "correct" to the apparent direction of gravity (as would be sensed by linear accelerometers)-- even for systems based not on physical gyros, but on integrating the outputs of rotational accelerometers?  So GPS input is un-necessary to a well-functioning AHRS system?  I sense another, more specific ASE question in the making here--

@quietflyer - Yes, mechanical attitude indicators work well without GPS or magnetometer, but they have substantial turning errors. You can get the same behavior with a strap-down setup. You cannot make the time constant too large because the rotation of earth is 15°/h and the erection mechanism must follow that rotation without too much lag. You also have gyro drift if you have cheap gyros and longer time constant means that drift can accumulate.

Hmm, considering asking a ASE question as to how quickly (and how large) errors would develop in actual practice in the event of permanent GPS signal loss to various types of actual real-world AHRS systems-- note these systems vary widely, starting with simple units intended to be plugged into your cell phone to allow better functioning of an artificial horizon app, to the most sophisticated military-aviation-grade units--

@quietflyer - I could give you some interesting input here. I've built an attitude indicator (for a drone) around a cheap ISM330DLC IMU, deliberately without GPS (unreliable) or magnetometer (you have to calibrate it first). The "simple averaging" approach works well, but you can do much more with just an IMU, including suppression of turning errors (but not acceleration errors from changes in thrust) and velocity estimation. The quality of cell phone gyros varies widely, some of them are okay-ish (ISM330DLC again), but others just suck (MPU-9250).

Fascinating -- may have some more questions later; this may end up going to a chat window-- thanks.  There was a time when I was actually extremely interested in making a very simple, light, portable, emergency attitude indicator for use in the event of accidental cloud entry in sailplanes, hang gliders, etc.  I ended up "addressing" the problem in other ways but still am rather interested in the topic--

What if we have a fighter jet that operates at excessive acceleration?
In this case is it possible to extract gravitation vector from the
accelerometer data? If we can't, that it means that at that operation
we only rely on gyro which will drift after a while.

A key point is that GPS and magnetometer data is used to understand the trajectory of the aircraft and allow the gravitation vector to be extracted from the linear accelerometer data.
As the original question rightly implies, if the rotational accelerometers were perfectly accurate, and if their outputs could be perfectly integrated over long periods with no computational errors (i.e. no computational "drift"), then this wouldn't be necessary-- in such a case, we wouldn't need need to make any reference to linear accelerometers at all, except during the initial start-up with the aircraft at rest.  We could note which way was "down" before beginning the flight, and simply "remember" that direction throughout the flight by continually integrating1 the output from the rotational accelerometers.  But that doesn't work in the real world.  We need to "help" things out by trying to also observe the direction of the gravitation vector in actual flight, and that's where the linear accelerometers come into play.
Here's a crude (oversimplified) analogy-- imagine that we have a mechanical artificial horizon gyro that for some reason drifts off from "true" rather quickly, and so needs to be mechanically reset to "level" now and then.  Imagine that we also have a simple pendulum (such as a weight hanging from a string, with some effective damping mechanism to prevent oscillations) that tells us the apparent direction of "down".  If we have a GPS, we can use the GPS track data to decide whether the flight path is linear or curving.2  If we only look at the pendulum when the flight path is linear and the G-load3 is exactly one, then we can sure that at that instant the pendulum is actually aligned with the real direction of gravity.  That's the instant that we want to re-set our mechanical artificial horizon to align with the apparent direction of "down".
Obviously, in the real world, it's a lot more complicated than that-- the correction is applied more continually, and lots of mathematical filtering is involved.
Note also that in theory, any "drift" in the integration routine for the rotational accelerometers could be corrected without any reference to GPS information.  After all, that's exactly how a mechanical artificial horizon works.  In that case, the principle is that the drift correction (to align the gyro with the position that a pendulum would take) is only applied to a very slight degree, so that during 90 or 180 degrees of turn, only negligible error has accumulated.  And once the aircraft has turned past 180 degrees, the additional "corrections" are in the opposite direction as the earlier "corrections", so after a full circle, the slight error induced by the "correction" to align the gyro with the position that would be taken by a pendulum4, has been fully removed.  But in actual practice, the computational "drift" involved in keeping track of orientation by continually integrating the output from a set of rotational accelerometers, is large enough that this would not be practical without some additional method of cross-correction, especially during vigorous maneuvering.
Footnotes:

Actual, a double integration is involved-- integrating rotational acceleration gives rotational velocity, and integrating rotational velocity gives the orientation of the aircraft in space.

It's inherent in the nature of GPS measurements, that it will be easier to detect curvature in the flight path in the horizontal plane (e.g. conventional "turns") than in the vertical plane (e.g. loops).

Here we mean the three-dimensional G-load, i.e. the net vector sum of the output from a set of three linear accelerometers.  As opposed to simply the reading on a simple mechanical G-meter (or a single-axis linear accelerometer) mounted on the aircraft's instrument panel.

See for example the reference to "pendulous vanes" at 4:01 to 4:12 in this video.  Food for thought: what erroneous "correction" would the pendulous-vane system apply to the mechanical artificial horizon, if the aircraft somehow accelerated horizontally at a very high rate for many minutes on end?

U_flow's answer is good too-- not sure why no u'vs in over a month.

It is not possible to extract the gravity ('downward') vector from accelerometers. It's a fundamental principle that acceleration and gravity are indistinguishable. If you place an accelerometer in a bucket and whirl it round your head it will tell you that the "downward vector" is outward from the circle. This is true of anything that measures only acceleration, no matter how sophisticated.
Other devices determine "down" by either maintaining a constant reference frame (a physical gyro) or very precisely measuring turning in all axes (laser gyro).
We have a Physics site where they can probably give you more details.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Feeling of falling during turbulence
I was on a flight recently that went through turbulence that had me convinced I was about to die. To be clear, I love flying. I find the sound of a pressurized air plane cabin to be very relaxing. I like to look out the window whenever possible, even during turbulence and it takes a lot to rattle me.
This was different.
The plane shook and bounced around in ways I had never felt before. It had bounced left and right and shuddered in ways that at first I just thought was sort of scary, but that's all. Suddenly, it felt like we were falling. It felt like we fell for a solid two whole seconds. I was thoroughly shaken.
The pilot got on the speaker and said a lot of things about being seated and putting on our seat belts, he said "safe" and "safety" quite a few times, and after the turbulence, he got on the speaker again and said he was communicating with someone to find a safe route and that we would be delayed. It felt like we were in a really dangerous storm or something.
Anyway, I'm curious if my fears were anywhere near reality. Did we really fall for those 2 whole seconds? How far did we probably fall? Sort of morbidly curious.

which flight was it? tracking it on flightradar or similar will tell you if it really was dangerous

Re, "...safe...safety..." Every so often you hear of people who were injured on commercial flights because they didn't put their seat belts on when they were told to do, and they hit their heads on the ceiling. (Or, hit other body parts on other things. I remember reading one account in the news that quoted a passenger. "Yeah!" he said, "People were just kind of flying all around the cabin.")

@SolomonSlow Yes, that's exactly it. The safety concern is from passengers going flying into ceilings or, more likely, stuff around the cabin (such as service items or passengers' items) going flying into passengers and cabin crew. The airplane is not going to be flying into any turbulence that would actually be dangerous to the airplane itself. Unless you do something really dumb like flying straight into a thunderstorm cell (which sane pilots would not do,) turbulence is not dangerous to airliners in cruise phase. Of course, it can be dangerous when very near the ground.

Other than confusion, what would be lost if you dropped all before 'Suddenly, it felt like we were falling…'?

@SolomonSlow: Or worse, the reckless people fall onto other people and injure people who did follow safety instructions.

It's possible. Two seconds of true freefall corresponds to about 60 feet of altitude loss. This is by no means unheard of- severe turbulence can sometimes result in hundreds of feet of altitude loss!
Most likely it was not true freefall, and it was somewhat less than two seconds. 10-20 feet of altitude drop is not rare at all and it feels like a lot more than it sounds like.
It's unlikely the plane was in any danger. While truly extreme turbulence could destroy an airliner, what you've described is nowhere close to that level. You were likely at some small risk of injury if some poorly-secured baggage were to fall on you.

An airplane is the only place I have ever dropped 20 feet.  I enjoy turbulence but that sounds like a long way to me.

@AnonymousPhysicist For comparison, the drop on Disney World's Tower of Terror ride is 130 feet. Almost any roller coaster will have significantly more than 20' drops.

One time I rode through moderate turbulence and was fearful when seeing the wings bend and bounce. Later I learned that the wings are engineered and tested for extreme bending, far more than what I saw on that ride: https://www.youtube.com/watch?v=B74_w3Ar9nI

@AnonymousPhysicist You're 20,000 feet up. What do you think you're going to hit in 20 feet?

Has anyone ever stopped to think that it could just have been the "simulator effect"? Cabin simulators move in a way that makes us believe we are in one type of movement, when in fact it is another. Cockpit simulators don't fall. A sharp downward acceleration followed by a very gentle stop will make you think you are still falling. As long as you don't look outside, you will believe this lie.
If you are really interested in knowing how you will die on a flight, take flight instruments with you.

An airplane is the safest form of transport in the world. Until it isn't. To your family, you are Schroedinger's passenger until you send them news when you arrive (or the TV anchor).
Until then you are paradoxically dead and alive. Flight thinking about this.

It is even possible the plane was not descending at all. Unless I am mistaken, an updraft can cause the plane to accelerate upwards and, if the upwards acceleration ends abruptly, it can feel as if you are falling even though you are really just moving upwards at a decreasing rate. If you put a pebble in a plastic bottle and toss the bottle in the air, you will see the pebble in free fall on the way up as well as down.
This is not to say turbulence is harmless. They want you to wear your seatbelts for a reason. The point is that the sensation of falling can be deceptive about what direction the plane is actually moving.

"The plane was not descending" gives no indication of the risk of injury.  A passenger who is not wearing a seatbelt could still hit their head on the ceiling when the plane is ascending but accelerating downwards.

Added a paragraph.

@AnonymousPhysicist True enough; but because the OP is wondering specifically about falling, with the implication that they feared they might fall too far (even if a simple back-of-the-envelope calculation shows that that would need more than 2 seconds): Falling is typically more dangerous for an airplane than climbing.

Just to be safe: please do not toss any bottle with pebbles inside a airplane. Especially during turbulence.

Paragliding pilot here.
A feeling of falling down or rising up is associated with vertical acceleration rather than velocity; so you might actually be losing altitude for two seconds or might have been in a strong updraft a second earlier and started to fall out of it.
It's nigh on impossible to calculate how much altitude the plane has lost in those two seconds, because you might be faster or slower than a free falling object due to vertical movement of the air surrounding the plane and the wings. If it's any reference, a free falling object would lose about 20m in two seconds, source here. I'm afraid it's also quite difficult to trust your senses and the perception of time at that moment.
What really matters is that very rarely do aircraft go through extreme turbulence and it's safer to keep the belt on throughout your flight if possible. As far as I know, aircraft pilots are benefitting from shared information with each other and control towers and can avoid turbulent areas as long as they're informed but every now and then a plane will get caught with no indication and no possibility of avoiding turbulence.

That sounds like the aircraft entered an area of moderate, possibly even severe, clear air turbulence that was concerning enough that even the flight crew wanted to divert from. Yes, there is a real hazard there. Fortunately, airplanes flying today are pretty darn well built and can take quite a beating from turbulence before they cry foul.  That doesn’t mean they’re indestructible but the danger areas are far beyond what you might think they are.

The main reason why the flight crew wants to escape from turbulence is passenger comfort, and the danger of spilled coffee in the flight deck.

SEVERE turbulence can accelerate a plane downwards at more than 1G, meaning anything not belted down "falls" upwards relative to the plane. I read that on one occasion a passenger's head went through the luggage bin above him, which (fatally) broke his neck. This degree of turbulence is  very rare, but a good reason to keep that belt on!

¨@nigel222 https://abcnews.go.com/Health/story?id=7391144&page=1 paralyzed because of turbulence (or, cinically said: because of not following flight-crew instructions ... sometimes we overblow our concerns about peeing&co, there is a reason why one should have always a complete clothes change in one's own handbag, a part from the possibility that the plane is diverted from sunny Los Angeles to the artic https://www.swissinfo.ch/eng/society/welcome-to-iqaluit-_swiss-plane-makes-emergency-snow-landing-in-canada/42930106 )

Just remember: the aircraft can survive more G's than you can.

Not necessarily true

@RuthLessPirate: not at all. Airliners envelope is always within -2/+5 g, and even less than that. Humans can survive twice that sustained, and much more on impact.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why aren't air turborockets used in military fighters?
Why aren't air turborocket engines used in jet fighters?
Few Third World countries have the capability to manufacture high quality turbine blades. Thus, why not use air turborocket engines for the time being?
Also, during World War II, rocket fighters like the Messerschmitt Me 163 Komet were developed. They were fast but since they excessively consumed fuel, their range was reduced. Using an air turbo rocket, it might be possible to increase the range of the Me 163 or similar such aircraft. So, why is nobody looking into it?
I feel air turborockets will be cheaper than a pure gas turbine in a fighter jet configuration. Why is no one pursuing this idea?

Why would they want to pursue an ineffective, outdated technology, that in the end will be more expensive than just buying modern stuff?

I'm not sure why you mentioned manufacturing turbine blades. An air turborocket also requires a compressor driven by a turbine, so if some country/company/whatever can't make good turbine blades, they're not going to be able to make air turborockets any more than they are any other kind of jet engine.

Of note: there are very few* ME163s being produced these days, so improving their range isn't a high priority. *i.e. exactly zero.

Why would 3rd world countries want to manufacture jet fighters in the first place?  Without even considering that there's a lot that goes into a jet fighter besides the engine, why bother when they can get them essentially for free from whatever 1st world country wants their support.

Listed under Benefits on the Wiki page link edited into your question:

The benefit of this setup is increased specific impulse over that of a rocket. For the same carried mass of propellant as a rocket motor, the overall output of the air turborocket is much higher.Emphasis added

Since most "jet fighters" use jet engines (or low-bypass turbo fans), not rockets this doesn't really seem to be a worthwhile avenue to pursue.
The same section continues:

In addition, it provides thrust throughout a much wider speed range than a ramjet, yet is much cheaper and easier to control than a gas turbine engine.Emphasis added

Where a ramjet is:

A ramjet, sometimes referred to as a flying stovepipe or an athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the engine's forward motion to compress incoming air without an axial compressor or a centrifugal compressor. Because ramjets cannot produce thrust at zero airspeed, they cannot move an aircraft from a standstill.From Wiki entry linked just below

There are very few ramjet powered planes, so the air turborocket would have a fairly small market when it goes looking for engines to replace.


text:
What type of bearings are used to support the shafts in jet engines?
I would like to know which types of bearings are commonly used to support the shafts in jet engines and why. I also read something about hydrodynamic bearings, how common are they?

I edited the title because it was incredibly broad and didn't jive with the question body. The question body is much more focused, though the hydrodynamic part would probably best be served as a stand alone question.

Rolling bearings

I would like to know which types of bearings are commonly used to support the shafts in jet engines and why

Rolling bearings are mostly of two types, depending on what's rolling: Cylinders or spheres:

Roller bearings to counter friction. They are used mainly for centering shafts within another component (e.g. a frame).

(Source)
We want to center the shaft but we don't want the shaft or the other part to heat, nor the rotation to be slowed down by friction. So we replace as much friction/sliding as we can by a part rolling between journals. It's like replacing sleigh skates by wheels between the ground and the sleigh, when the ground is not snowy.

Ball bearings. In addition of acting against friction like a roller bearing, an angular contact ball bearing also prevents any longitudinal movement between the shaft and the other part. Thrust for example pushes longitudinally on shafts which would slide out of the engine if not blocked. The roller bearings act as blocking elements, shafts remain at their position withing the engine, and thrust is transmitted to the wing via the bearings, the frame and the pylon.

(Source)

You can see roller (magenta squares) and ball (circles) bearings in this engine schematics taken from In a turbofan what holds the spinning axis:

Frames, shafts, bearings. Adapted from CFM56-7B Familiarization Manual
What we see on this design:

Two ball bearings are used to give N1 and N2 shafts their longitudinal position relatively to the fan frame.

These ball bearings also center the shafts within the frames, allowing rotors to be centered in the engine walls.

Three roller bearings are used to center the shaft at other selected locations, due to the shaft length and loads.

Other bearings are used for the gearboxes which connect the N2 shaft to the accessory gearbox, visible here:

Source: Where is the generator in a large turbofan of a commercial airliner?
(There are many other types of rolling bearings with needles, tapered rollers, double row, ... each having some advantages in a particular use.)
Hydrodynamic bearings

I also read something about hydrodynamic bearings, how common are they?

Hydrodynamic bearings reduce friction, not by rolling parts like balls or a rollers, but by injecting oil between the rotating elements, friction is decreased, only oil viscosity tends to slow down the rotation.

(Source)
It's the traditional bearing, with a lubricant, like waxed skates.
The "dynamic" part is because this is the motion of the rotating part which creates the oil film, using the same principle than for accelerating air over a wing: Shaft velocity creates a boundary layer in oil. After the film is created, the motion is no more between metal and metal, but between metal, oil and metal.
Their main advantage is they can bear large loads and still be accurate in positioning. You can read more this technology here. They are commonly found in rocket turbo-pumps. Here's one which is able to bear a load along the longitudinal axis, borne by the flat pads.

(Source)
Actually roller and ball bearings also use oil to minimize rolling resistance. The difference is rolling versus sliding. Rolling is usually more efficient to decrease friction effects than sliding since the area at the interface (metal-metal or metal-oil-metal) is reduced.

And what is the purpose of the final figure of a thrust bearing ( tilting pad ?) ?

I agree with @blacksmith37, the tilting pad thrust bearing seems out of place here. They are rarely used in jet engines (partly because there is not that much thrust on the shaft: most of it is cancelled out between the turbine and compressor). It's just an opportunity to repeat common misconceptions: for example, these bearings can be more efficient than roller bearings, particularly under high load.

@blacksmith37: The OP asked about what is a hydrodynamic bearing. This answers this point, even if they are not used in the engine shown.

It's much better now, although the last sentence is still misleading: the area has almost nothing to do with it. And in the explanation of hydrodynamic bearings, I would add one perhaps most important clue: they don't just form a "boundary" of fluid between the surfaces; it is hydrostatic bearings that rely on a plain boundary. Hydrodynamic bearings form a wedge of fluid; this is why the shaft is shown off-centre on the picture and why you need tilting pads on the thrust bearing shown. They indeed behave a bit like a wing. It is none other than Osborne Reynolds who created this theory.

@Zeus: You would save time by writing your own answer, free of misconceptions.

@mins, why? You've got a good and complete one; I just feel it could be improved and made excellent.

@Zeus: I thank you for that, but I don't feel you're correct in "they don't just form a 'boundary' of fluid between the surfaces", I didn't say this is boundary lubrication, a specific wording in ME. I told about a 'boundary layer', that is a gradual velocity change in the oil wedge, which indeed exists by viscosity effect, one side of the wedge moving with the shaft, the other being stationary, resulting in the pressure increase in the converging nozzle, peculiar to hydrodynamic lubrication.

You don't see hydrodynamic bearings in aviation very much. Jet engines are generally ball and/or roller, and piston engines with machined one-piece crankshafts use plain babbit type bearings that depend on pumped oil pressure to separate the metal of the bearing and journal (piston engines with multi-element cranks may use ball and roller bearings).
Another bearing you see in components like Air Cycle Machines (used in air conditioning packs) are "air bearings", where a gas is injected into the journal to create sufficient pressure to separate the metal parts; more or less a plain bearing but with air instead of oil.  Air bearings require a conventional low friction bearing that supports the shaft during startup until the gas pressure becomes sufficient.  Once the pressure is sufficient to lift the shaft clear, these bearings are near frictionless.
The CRJ700 and 900 Air Conditioning packs use Liebherr Aerospace air bearing ACMs that pressurize their bearings using main engine or APU bleed.

Why wouldn't a Babbitt bearing on a connecting rod, main, or cam bearing not be a hydrodynamic bearing?

A hydrodynamic bearing works passively, using the motion between the surfaces and the ramp effect to build up a pressure wave of oil.  Babbit bearings are helped by ramp effect, but under high stress they are dependent on oil pressure forced in from the pump, and a larger inlet than exit area, to keep the surfaces apart.   A plain babbit bearing once pressurized has an infinite life in theory, as long as oil pressure is adequate. So oil pressure below the green arc on takeoff on a piston engine means your bearings are being destroyed by metal contact, but at or near idle, not such a big deal.

oil is injected between the bearing and shaft. The oil wedge created (the oil between the bearing and shaft

From https://www.sciencedirect.com/topics/materials-science/hydrodynamic-bearing

oil is injected between the bearing and shaft. The oil wedge created (the oil between the bearing and shaft at the load point) is only about 20–25 μm tick, about the thickness of a human hair.


text:
What is output by the Seminole's twin alternator system?
The Seminole's alternator system is a 14V 70 amp system. I understand that if one alternator fails, the other one will be able to produce 14V and withstand 70 amps.
When both alternators are operative, will the system be able to produce 140 or 70 amps?
I guess just 70 but want to make sure!

If those numbers are correct, and typically alternators like that are going to be wired in parallel to the electrical buses in the aircraft, that means, with both alternators operating, they can deliver a maximum of 140 Amps at 14V DC to any load drawing from the electrical buses, provided the fuses on the individual buses are capable of handling those kinds of loads.


text:
Is there a list of part 121 and/or 135 operators?
While looking into this question, I found a list of certificated air carriers on the US Department of Transportation website (there's a list of commuter air carriers too). But, the list doesn't say if each one operates under part 121 or part 135, which the DoT seems to consider an FAA question anyway:

As noted above, the economic authority issued to air carriers by the
  Department is separate from the safety authority (commonly referred to
  as Part 135 or Part 121 Operations Specifications) granted to them by
  the Federal Aviation Administration (FAA).  For safety authority
  application procedures and related questions, air carriers should
  contact the appropriate FAA flight standard district office based upon
  the geographical location of the air carrier

The FAA site has a lot of information about part 121 operations, but at least as far as I could see there's no list of 121 or 135 carriers anywhere on their site.
Is there a list somewhere of current part 121 and/or 135 operators?

From FAA website, there is a list of Air Operator to download. You can choose the format you like and open it in your favourite spreadsheet. How to delimit the file depends on your program and out of scope here.

I have not found a list, but you can search for an airline here.
There is a link on the page to definitions of the terms. Basically, "domestic" and "flag" are FAR 121 operations, and "commuter" is FAR 135 operations.
There is also an unofficial database here that will tell you which FAR an airline operates under.

Here is a list of every Part 135 aircraft registered and their operators.
https://www.faa.gov/about/office_org/headquarters_offices/.../135Aircraft.xlsx

Please re-post the link... what is on the page doesn't work.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why do turbofan jets fly faster in cold weather?
Why do turbofan jets fly faster in cold weather at high altitudes?
I read somewhere that prop planes do it due to cold weather means air is more dense, and the prop therefore produce more power.
But what about jet aircraft at say 36000 feet? I looked at performance chart of a jet fighter, and at ISA-15 it could fly 0.17 mach faster than ISA.
Do not misunderstand I am NOT asking why it flies faster higher up. I am asking why it flies faster at winter than in summer at same (optimal for speed) altitude.

The question is, do they really fly faster? The speed of sound goes down with lower temperature, and it is cold at 36,000 ft

Can you post an image of the chart? I believe if the shape of the max speed curve vs altitude (assuming that is what the chart shows) has the same shape as a curve of constant dynamic pressure, it would indicate the max speed limit is a structural limit. If not, the limit is due to another constraint (probably engine thrust?). These two limits also tend to occur for different altitude ranges. So knowing the shape of the max speed line might be very useful to answer the question.

They don't. They fly slower.
Most aerodynamic effects depend on dynamic pressure. More precisely, lift is proportional to dynamic pressure and drag has parasitic component, which is proportional to dynamic pressure, and induced component, which is inversely proportional.
Therefore there is an optimal dynamic pressure at which the aircraft flies most efficiently, and minimal dynamic pressure needed to maintain flight.
Now dynamic pressure is (half of) air density times velocity squared ($q = ½\varrho v^2$) and due to its significance the “indicated airspeed” shown in cockpit actually corresponds to dynamic pressure expressed as equivalent airspeed. All the desired speeds for various phases of flight are referenced to that.
Since cold air is denser, the same dynamic pressure will occur at lower velocity, and because the pilot will maintain the same indicated airspeed, they will actually fly slower.
One more thing: the maximum speed is one of the few exceptions that is not dependent on the dynamic pressure. There are three effects that may limit maximum speed:

Aeroelastic flutter depends on true airspeed (velocity), so the temperature does not, actually, have an effect on it. However because true airspeed is not usually indicated in cockpit, the pilots will use tabulated indicated airspeed values and since these are only tabulated for altitude, they will actually fly slower.
Mach tuck i.e. loss of lift, accompanied with undesirable change in trim, due to exceeding critical Mach number. Since speed of sound increases with temperature, critical Mach number corresponds to lower true airspeed (velocity) in colder weather, so they'll fly slower again.
Engine power. Engines are able to produce more power in colder weather due to being able to achieve higher mass flow with the higher density air. However, engine power is limiting factor mainly for smaller general aviation aircraft, which have low power engines, or fighters at the other end of the performance spectrum, which are designed to fly supersonic and resist flutter. But transport category aircraft generally hit the other two limits first.

Original answer for the version before the reason they fly at higher altitude was explicitly excluded from the question:

There are two, distinct, effects:

Aircraft fly faster at high altitude due to the lower air density. This is because both lift and drag are proportional to air density. So at higher altitude, the plane both have to fly faster to generate enough lift, and can do it because the drag is lower.
More precisely, lift is proportional to dynamic pressure and drag has parasitic component, which is proportional to dynamic pressure, and induced component, which is inversely proportional.
Therefore there is optimal value of dynamic pressure where the drag is lowest. The optimal cruise speed is slightly above this point. Since dynamic pressure is proportional to density and square of speed, the same dynamic pressure corresponds to higher true airspeed at higher altitude.
Dynamic pressure is usually expressed as equivalent airspeed, which is the speed to which current dynamic pressure would correspond at sea level. The indicated airspeed shown on the instruments is approximation of this value.
Heat engines are more efficient at lower temperatures. This is because efficiency of a heat engine (applies equally to both piston and turbine engines and both internal combustion and external combustion (steam) ones) depends on the ratio of temperature, but the combustion increases the temperature by the same (approximately) difference. So if the starting temperature is lower, the ratio is higher and the engines produce slightly more power for the same fuel flow.
Additionally, the maximum temperature is often limited by what the materials can withstand, so at higher altitude the engine can be operated at higher power (well, not really; the power is also proportional to the density, but the engine will again run more efficiently).

There is more to it: Low temperature means there are more Mols of oxygen around to react with the fuel and, most important, there is a bigger temperature margin until the max T4 is reached, so more fuel can be burned. This is the main reason for the higher thrust. It does not mean, however, that the aircraft is more economical to operate - only that more thrust can be had if desired.

@PeterKämpf, Mols are proportional to density, no? And since I used density as a reference throughout (because equivalent airspeed also depends on density), there is no additional effect.

@PeterKämpf, also, if the margin until max T4 is higher, and is taken advantage of, the thermodynamic efficiency—and therefore  specific power—does increase.

Not anywhere near that complicated. Newton's third law: Conservation of momentum. Cold air weighs more than hot air. If you are sucking in more air (by mass) the engine is expelling more mass out the back. More mass means higher momentum equates to higher thrust.

@CharlesBretana, I was about to explain how that says exactly nothing about efficiency, but then it made me actually re-read the question and replace the whole answer.

(@CharlesBretana, I was originally answering before the altitude part was explicitly excluded)

@Jan, No worries, I must have read the question after he added that part...

Engines are limited by maximum temperatures/pressures. Above a certain limit either you exceed short term or long term safety margins.
When the incoming air is colder, you should be able to increase your fuel flow slightly because the extra fuel mass still keeps combustion temps within margins, because they start with colder air/fuel.
Also by having colder air coming in, the same fuel flow produces more thrust/power because of higher thermodynamic efficiency.
https://en.wikipedia.org/wiki/Thermal_efficiency
Look at brayton cycle for turbines.
Piston engines are typically installed so max throttle keeps a reasonable safety margin even on a hot day, so you can't give it a little extra fuel in a cold day for takeoff, you only get the benefit of thermodynamic efficiency (and a little leaner combustion because denser air is coming in for the same max fuel flow).
But turbines are to be operated by professionals or controlled by FADEC (full authority digital engine control). Meaning if there's no FADEC the pilot must limit the throttle manually to a target temperature limit.
For instance I flew quite a few times on Cessna 208 Caravans (non FADEC turboprop) on the right seat, just as a private pilot rated passenger. Takeoff throttle had to be hand limited at a given max temperature. As the aircraft climbed, throttle was periodically added to compensate for altitude (honestly I don't think it really was because of altitude but something to do with OAT, otherwise the throttle would have to be reduced with altitude instead). This method naturally leads to a higher fuel flow under the same altitude the colder OAT (outside air temperature) is. On bigger aircraft FADEC takes care of that, you just command the required detent for takeoff (TOGA), MCT/climb (max continuous thrust). The point being that FADEC will command higher thrust for take off using TOGA on a colder day than on a hotter day, to comply with temperature margins, same for other flight regimes.
But... Cruising speed at the same altitude for a jet is highly dependent on if the engine still has excess thrust left or not ! If you are below the altitude where cruise will be throttle limited, do you really get faster cruise on a colder day, or is it only high enough where the engine is running at MCT and the colder air produces more thrust naturally ?
PS: I'm not an expert on the subject. I have not received turbine training. But I am a private pilot with IFR and have college level physics which allows me to understand all concepts involved. Maybe I shouldn't be posting an answer here at all. Let me know.

It is simple physics. The law of conservation of Momentum (Newton's third law I believe) says that for each action there is an equal and opposite reaction.
The total momentum (Mass times velocity) must remain constant.
So if you throw a cast iron anvil out the back of the ship, the ship will move forward faster than if you throw a feather out the back at the same velocity.
Cold air weighs more than hot air. A jet engine operating at high density altitude (when air is very warm) is sucking in significantly less air by mass than a jet engine operating at a lower density altitude (colder) at the same pressure altitude.  Higher mass of air sucked in means more mass is being shot out the back. So the Thrust is higher.

Nope, Thermodynamics is what you need to look at. Far, far more complex than you think.
In fact the denser air argument is a net negative because the drag effect of thicker air on the airframe will be more significant than having more mass to push on the engine.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Did WWII fighters have inverted airfoils for horizontal stabilizers?
Like would for example the P-47 most likely have symmetric, normal or inverted airfoil as horz. tail?
Is my analysis correct?:
It has a CG at 25-32% MAC according to manual. Since AC on wings is usually located around 25% MAC it safe to assume the CG is aft of AC. Since the setup is that way, it would be normal to have horizontal tail producing positive lift.
Also; did typical WWII planes have inverted airfoils as horizontal stabilizers?
What about general aviation planes today?

The P-51 used a NACA 6-Series airfoil which was not symmetric


text:
Can a taildragger land tail first?
Inspired by this question...
How does a taildragger take off? Does it rotate?
I imagine if a taildragger has to land front wheels first it has a very limited range of Angle of Attack (if the pilot wants to avoid a prop strike) during landing.
So do they land tail first by default?
If not would a tail first landing be an issue?

I assume you mean with the tail touching the ground first, not that the tail is in front of the mainwheels. Flying backwards is not typically possible!

@DanHulme I did come to the conclusion that the tail had to be behind the nose by myself yes! Glad you said typical too! The Harrier and F35/JSF do like to show off with that.

@Notts90 That and the Antonov An-2.  Which is a tail dragger that, in a stiff wind, could probably land tail wheel first.

@DanHulme, I saw a floatplane fly backwards once (slow plane in a strong headwind), but I wouldn't want to land in those conditions.

It's possible to land by touching the tail first, but this is undesirable as it puts a lot of stress on the tail, so it's not trained for and there are no circumstances where you would do this deliberately. There are two common methods of landing a tail-dragger: a three-point landing (where the tail and mainwheels touch together) and a wheel landing (where the mainwheels touch first).
Aircraft design
First, be aware that the aircraft is not level when it's on the ground, like a tricycle aircraft is. The tail position is set so that the aircraft is quite nose-up on the ground - in fact, the angle is about the same as the stalling angle, and the reason for this will become apparent. If your nose is on the horizon, the tail is off the ground (about five feet up in a Tiger Moth). Also, the prop is mounted higher up on a tail-dragger than on a tricycle, for exactly the reason you say. In the Tiger Moth, there is two or three feet of clearance between the ground and the prop if the nose is on the horizon.
The clearance itself isn't the direct problem. If you do go nose-down on the ground, the airplane is not stable. If you're moving forwards, the chief component of drag is acting through the mainwheels, so the engine thrust will tend to tip you further nose-forward (especially if you run over a hump or rabbit hole). In addition, the CG of the aircraft is closer to the mainwheels, and eventually goes in front of the mainwheels. If this happens, a prop strike is inevitable.
Three-point landings
Three-point landings are the default because the aircraft is fully controllable as soon as it touches down. The approach to the runway is done at low or idle power, and set you up to be flying level at rooftop height above the runway. As you lose speed (because you have idle power), you progressively pitch up to maintain height, which in turn causes you to lose more speed. This is called the flare Soon enough you are pitched up at the three-point attitude, and you can no longer maintain height - if you pitch up any further the wings will stall. At this point, you will drop down onto the ground and all three points will touch at once.
Wheel landings
Wheel landings are also used, as they offer more control in windy conditions, but they typically have a longer landing roll because you have more speed when you touch down. The approach is flown with some power on, but less than cruise power. It's a gradual descent towards the runway, and there's no flare, so you touch down in the level attitude. You then close the throttle to idle, and let the speed decay on the ground. As your speed decays and less lift is produced, you need to progressively push the stick forward to keep the tail from dropping, keeping the aircraft level as long as possible. Once the tail starts to drop, pull the stick back to drop the tail firmly onto the ground.
When do tail-first landings happen?
To reiterate, you never land tail-first deliberately. It offers very little control; the aircraft would stall at a higher speed (and therefore the landing roll is harder and longer); it puts undesirable load on the tail. They do sometimes happen by accident, if a student pilot has pitched up too aggressively when trying to do a three-point landing. Typically, too much nose-up input results in "ballooning" as the aircraft gains a little more height, but this results in the speed decreasing rapidly. If left uncorrected, the aircraft stalls, resulting in the tail touching down hard.

When I was learning to fly, we did practice "soft field" (or "low-energy") landings. The scenario is that you need to land in a mucky field or on snow or some other soft surface and if you touch down main wheels first, you run the risk of putting it on its nose, or at least striking the prop. You come in as slow as possible (just above a stall), and touch down tail then wheels, in rapid succession. You're really just plopping it down at as slow a speed as possible, and doing your best to avoid prop strike. This is probably not a common thing they train for, but I grew up flying small airplanes and sailplanes in Vermont, where if you're landing out somewhere, if it's not trees, it's probably muddy or snowy.

I have been flying a Maule for many years and do mostly three point landings, and sometimes wheels only.  However when landing at an airport with say an 8,000 ft runway (which I hate to do as I have 29" Bushwheels) I will sometimes land with some power on and just touch the tail wheel down to 'gobble up' the runway to the exit point and then let the main gear touch and dump the flaps. This saves rubber on my Bushwheels and allows me to stop fast.

A tail landing isn't really a landing. Not until the main wheels are down.
The main wheels are designed to support the entire weight of the plane, and often do. You could, at least in principle, put down the main wheels and keep the tailwheel off the ground until the plane is well below flying speed, maybe until it's nearly stopped. (I'm not recommending it, but it's possible.)
By contrast if you put the tailwheel down there is no reasonable way of keeping the main wheels off the ground below flying speed. The tailwheel won't support the weight, and the plane would be unbalanced both in pitch and roll. Until the mainwheels are down the pilot has to keep flying the plane.
To summarize: you haven't really landed the plane until the mainwheels are on the ground.
So basically putting the tailwheel down first gets you no benefits whatsoever over the other methods. When it does happen, it's by accident.

"Until the mainwheels are down the pilot has to keep flying the plane." You're not wrong, but the pilot still needs to keep flying the airplane once the mains are on the ground, especially in a taildragger. The question isn't "can an airplane land only on the tailwheel" but "can an airplane land with the tail touching down before the mains".


text:
Are there pilot weight limits for the Rutan Vari Eze?
In this question on the Rutan Vari Eze it is said one of the reasons it parks nose down (gear up) is to bring the Cg forward for stability (so it doesn't tip over) when there is no pilot in the aircraft.
If the pilots weight is that important to move the Cg to a safe position, is there a minimum/maximum weight the pilot must be to bring the Cg to be within acceptable limits?

I'm not sure about minimum, but there is always a maximum.

A minimum wouldn't be surprising either. Such limitations are very often (always ?) encountered on gliders to keep the Cg where it should be. Pilots must add some lead in the cockpit if they are too light.

A lot of gliders and other light aircraft have minimum and maximum weights, and will need extra ballast added for light pilots.

According to the Owner's manual,

The air plane can structurally accommodate pilots or passengers weighing up to 250 lbs.

Of course, this is subject to the maximum allowable takeoff weight.
The manual also gives the forward and aft cg limits (in page 30). In case of very light pilots, the aft cg limit may be exceeded and it may be required to carry ballast for bringing the cg within limits.

Thanks for the link to the manual, it actually gives a "light pilot" (135 lb / 61 kg) example that puts the Cg just inside the limits. So I guess this is effectively the minimum pilot weight before ballast is required.

@Notts90 Your guess is generally true, in that a generic Vari Eze—as represented by the generic owners manual—will require a minimum weight at the pilot station of 135 lbs, or an equivalent moment produced with the help of ballast elsewhere. However, understand that every individual aircraft has a different weight and balance, and so the minimum weight required to keep CG in limits will vary by individual aircraft. For this reason it would be improper to state categorically that 135 lbs is the minimum pilot weight for the Vari Eze.


text:
How does flying with flaps differ from flying with airbrakes?
I've got a private pilot license and learned on a Scheibe Falke SF25. For the last year however I've only been flying a C42 Ultralight.
I find flying the UL way easier for one particular reason: it has flaps where the Falke has just airbrakes.
I don't know how many hundred landings on the Falke I have done, still I have never felt comfortable. The reason is the airbrake. For some reason it scares me. With the UL I set the speed, lower the flaps and if I'm too far away I can add a little power. The flaps lower the stall speed and make the plane slower.
The airbrakes also make the plane slower, however they increase the stall speed. I see how (especially flight instructors) pull the air brake to the maximum and then do their approach. If I'm too far away with the Falke, I have to retract the air brakes. This affects the plane very directly, especially compared to adding a little power on the UL.
My questions are:

How to correctly land a plane with airbrakes? Full out or just a bit? Do you leave them in one position or do you vary during approach?
How much do they really affect stall speed?

Airbrakes (aka spoilers) increase drag and decrease lift while flaps increase drag and increase lift.

"decrease" and "lower" are (near-)synonyms; I suspect you meant "decrease lift" or "increase the stall speed".

It should read "airbrakes increase stall speed".

@PeterKämpf True. Fixed.

You don't say if you have the engine running or not during landing.  Also, for proficiency's sake, I would go up with an instructor and spend an hour doing airwork with and without the airbrakes.

The falke has pretty poor spoilers. Opening them fully has nowhere near the effect of the airbrakes found on most gliders. The falke is also very susceptible to bouncing when landing with too much airspeed since the centre of mass is quite far behind the wheels. All of our pilots learn to fly gliders first, then transition to the falke and all find it hard to land. It's a taildragger, so it's a bit unfair to compare to tricycle. You're right that the C42 has a lower stall speed with flaps, but I think you'll find the spoilers don't affect the stall speed at all. Try it!

The SF-25 has such a low wing loading that it easily tolerates the small reduction in maximum lift which the airbrakes cause. I have done very few flights in it (I'm too tall to fit comfortably in it's cramped cockpit), but from what I remember the airbrakes are not very big, having little effect on minimum speed. My experience, however, is with the SF-25 B, and you linked to the SF-25 C, so I have to assume that my experience is not fully relevant.
I have more experience with real gliders and composite motorgliders, and here my experience is: Start your approach, stabilize the right approach speed, set the airbrakes such that your glide path looks right and do not play around with them too much. Set your approach such that you need medium airbrake extension, so you have margin for corrections in both directions. Normally, I come in a little high so I am not caught by windshear, and when I am sure I will reach the intended touchdown point, I extend the airbrakes fully to land with the shortest distance possible. However, when touching down, make sure you do not fully extend the airbrakes: This gives you more time for stopping the descent. Especially in gliders with very effective flaps and airbrakes (ASW-20 C, ASH-25) I never made a good landing with fully extended airbrakes. Reduce them to half extension at the most when touching down!
In the ASW gliders the airbrakes are linked to the very effective wheel brake, and touching down with fully deployed airbrakes means that the wheel is braked at touchdown. This is another reason to reduce airbrake extension when touching down.
In high-performance gliders it is easy to float too long, so it is very important to control your approach speed and to continue the descent all the way down to the touchdown point. Don't pull too early! The SF-25 will not float as much, but with its low wing it shows a pronounced ground effect, and exercising strict discipline when landing helps a lot to make the landing better.
I could not find a number for the stall speed change due to airbrakes in the SF-25 manual, but I would guess it is at most a few km/h.

One thing to mention regarding flying gliders in general (I have never flown the SF-25) is that you should maintain proficiency in performing slips so that you assure your landing spot when you're so high that full airbrakes on approach is insufficient.  And remember, great approaches make great landings.

I would not recommend flying an approach to land with maximum airbrakes.  Instead, what I was taught to do in a glider (or a motor-glider without the engine running) is to plan and fly the approach with about 1/2 airbrake extension.
This allows you to use the airbrake handle much like you would use the throttle in an aircraft with a running engine:

Getting a little low or slow - Retract the airbrakes as far as needed to return to your normal approach angle.
Getting a little high or fast - Extend the airbrakes as far as needed to return to your normal approach angle.

You then touch down with whatever airbrake extension was required, and then fully extend them afterwards.
Flaps are unrelated to airbrakes, and you can have gliders with or without either of them.  Some gliders have both!  The flaps allow you to slow further, which reduces your ground roll and also increases drag.  This is important in high performance gliders because it can be very hard to actually make them go down without gaining speed when you want to land.
As far as how much they affect the stall speed, this is very much dependant upon the specific aircraft and the design of the airbrakes/wing.  The POH should describe any special procedures (such as flying your approach speed + 10 with full airbrake extension) if they are needed.

It's bad to compare flaps and speed brakes and use flaps as brakes. They are meant for different things. To slow down and to make stall speed slower. Of course it's possible to use flaps as speed brakes but it's a bad practice. Boeing 737 flight manual warn's pilots not to use flaps to slow down. High performance aircraft usually has both. Another important think is that speed brakes doesn't have speed limit. Flaps have speed schedule and are meant only for slow speed. For example you want to descent fast you maintain maximum speed and extend brakes.

Split flaps make great landing aids because they simultaneously increase lift and drag. A lot. Flap speed limits are discussed here

Flaps and spoilers(airbrakes) have a similar affect by creating more drag. However, they have the opposite affect when it comes to creating lift. A great way to think of this is the car analogy. There are two ways to slow down a car: the hydraulic brakes and the engine brakes. Hydraulic brakes, analogous to spoilers/airbrakes, decreases revs as it slows down the vehicle. On a standard transmission, applying the brakes without hitting the clutch will eventually cause revs to go to low and stall the car. Similarly on an aircraft, using spoilers will increase the angle of attack(decrease lift) and of course increase stall speeds.
Conversely, flaps work like engine brakes. If I'm cruising at 50 mph(80 km/h) and my revs are at 1500 rpm in 6th, and I put my car in 5th while maintaining the same speed, I will have to increase the power I put and my revs will jump to about 2000. SAME CONCEPT IN AN AIRCRAFT! If I'm cruising at 180 knots IAS and my angle of attack is 10 degrees, and I apply flaps from up to 10 degrees, the engine will have to use more power to maintain 180 knots IAS, but my angle of attack will decrease, and I'll get more lift. Just like a car has speed limits for its gears, aircrafts have maximum speeds to use every level of flaps. Using engine brakes can help you slow down just like flaps can help you fly at a lower speed and eventually land at an optimal speed.

increase the angle of attack(decrease lift) that's wrong, increasing the AoA increses lift (up and until the critical AoA is reached)


text:
Do coaxial rotor helicopters have a size limit?
Why haven't there been heavy helicopters like the CH53 with coaxial rotor configuration? Is there any physical limitation inherent to the concept?
I recall someone at Sikorsky saying that their new X2 coaxial helicopters that they built for FARA competition, etc. don't scale up to heavy helicopters. But it is likely that they only meant the high speed compound mechanic and not coaxial rotors as a whole.

Does this answer your question?

So what I understand from this is that the difficulty in creating enough differential pitch with increasing interaction between the rotors and the bending forces on the rotorshaft while experiencing dissymmetry of lift are the reasons? Am I getting that right?


text:
Why are the FAROS and the RIL contents removed from AIM?
I was studying runway status light and I found that FAROS (Final Approach Runway Occupancy Signal) and RIL (Runway Intersection Lights) are deleted when I read FAA AIM. It is deleted in the FAA AIM CHG 3(12/31/2020).

2−1−7. STAND−ALONE FINAL APPROACH RUNWAY OCCUPANCY SIGNAL
This change deletes verbiage to Final Approach Runway Occupancy Signal (FAROS) and
Runway Intersection Light (RIL) systems content, including revision of
figures representing their removal.

Is there any reason for it? Does the FAA no longer use those systems?


text:
What is a spool in any jet engine, what is its funtion?
I am a student and I'm trying to determine how a spool works in an engine. What effect does it have depending on the number of spools on the performance of the engine?
I tried to find it on Google but did not got any solid answers.

Start with the background: in a jet engine the air flows through the inlet into the compressor (to increase pressure), then through the burner (combustion chamber) to increase energy, then through the turbine (to extract power to turn the compressor) and finally out through the nozzle to provide thrust. In a single spool engine, the compressor and turbine stages are connected by a single shaft - so that is a one spool engine.
High compression ratio is needed for efficiency and it is difficult to increase on a single shaft so the dual spool engine was created. The air goes from inlet to the "low pressure compressor" then the "high pressure compressor" (LPC and HPC). The turbine has stages attached to each compressor (the HPT and LPT).

Does the lpc and hpc rotates in different directions to each in 2 and 3 spool engines, and Does the shaft drives spool ?

Spool is a term to  link the compressor, turbine and the shaft that connects them. Generally, the rotation is in the same direction. For example, at full thrust the F100 in the F-15 or F-16 has the fan at N1= 10 000 rpm and the HPC at about 12500 rpm so the difference is only about 2500 rpm. If they were counterrotating, some one would have to design and build real high speed bearings. They have, the F119 in the F-20 is similar  in size but is counterrotating. The gyroscopic effect should be better in the latter.

A spool in a gas turbine consists of a compressor (or fan), a connecting shaft and a turbine, see this answer. The purpose of the spool is to compress air driven by combustion gasses through the turbine. If not all energy is used in the turbine, it is either used in the next turbine or expanded in an exhaust nozzle.
The image below depicts in dark/light blue the high pressure spool (typically called the N2 spool), on the left the individual compressor stages that form the High Pressure Compressor, HPC, in the middle a connecting shaft piece (or drum) which connects to the turbine. The light blue shaft parts are also known as hubs and form the start and the end of the spool:

HP rotors of CFM56-7B. Adapted from CFM56-7B Familiarization Manual
The rationale behind using multiple spools is given by the type of gas turbine and the maximum compression ratio. To increase the propulsive efficiency, large amounts of flow need to be accelerated a little, this leads to large blades (typically called the fan) which may not be rotated too fast to prevent the tip speed to reach the speed of sound (for the sake of losses and structural integrity). Furthermore, given the fact that gas turbines operate at different power settings than the design value, where the axial flow through the machine is constant, a lower power setting will cause less fuel to be added to the cycle and as such less power generated in the turbine to drive the compressor resulting in a lower spool speed and thus lower compression ratio. This (less) compressed air is in volume larger than the design (high power) situation and will therefore increase the axial speed. This increase of axial speed causes blades to stall unless measures are taken (variable stator vanes, or air bleed). A compression ratio of 8:1 is typically the maximum for a single spool. To obtain higher compression ratios, to increase the cycle efficiency, multiple spools can be added to mitigate the problems at part power.

I like Oscar's answer about what is a spool. I'd like to add more information on the number of spools
Going from 1 spool to 2 spools takes you from a turbojet (1 spool) to a turbofan (2 spools).  There are many good existing questions on this site regarding turbojet vs turbofan so I won't go into that.
2 spools vs 3 spools is different.  Both configurations would be turbofans, with one spool being the fan, aka a low pressure compressor, driven by a low pressure turbine. The fan generates the majority of the thrust of the engine. The remaining 1 or 2 spools form the core of the engine, which generates the hot gas flow necessary to power the low pressure turbine.  GE generally chooses 2 spool designs, whereas Rolls Royce generally chooses 3.  The difference is a tradeoff between mechanical complexity and aerodynamic complexity.
With a two spool design, there is only one spool for the high pressure compressor.  Each individual compressor stage only gets a small compression ratio, so to get the overall compression ratio that you want, you need to stack multiple compressor stages together, say 9 - 11 of them.  Now, at one particular design condition, say cruise, every single stage can be completely matched to work together perfectly.  But at any other design condition, say takeoff, the stages aren't perfectly matched. The front stages might be pumping more air than the back stages really want to handle.  You'd really like it if you could slow down the front stages a little bit and speed up the back stages.  But you can't, they are all one one shaft.  So make it work (with good efficiency, good stall margin, etc) you have to play all kinds of games with the aerodynamics: variable geometry like VSVs, bleed valves, very detailed CFD models, etc.
Now with the three spool design, you have two spools for the core.  So instead of single high pressure compressor, you can split that into an intermediate pressure compressor and a high pressure compressor.  So now you have more flexibility. On those off design conditions, you can slow down the front stages a little bit and speed up the back stages, because they are on different spools, so you don't need fancy aerodynamic design.  But the price you pay for that is mechanical complexity.  Every spool is a rotating shaft that has to be supported by bearings.  So now you need more bearings.  Bearings have to be supported by something, so now you need frames and internal supports to hold them, and they need lubrication, etc. All these parts need to be bolted together somehow. Overall you have a lot more parts to the structure.

Are you sure turbojets are always 1 spool? AFAIK, the Olympus 593 that powered Concorde was a 2 spool design.

@Bianfable Also the P&W J57, a 2 spool turbojet. But, I don't think there are many.


text:
Why does the B2 bomber's trailing edge sawtooth shape negatively effect its stealth characteristics?
I read an article about how the new B21 would have (in theory) a W shaped trailing edge, which was the initial shape intended for the B2 before the gust alleviation system was added to the requirements causing it to have the sawtooth trailing edge.
From what I see from the planform of the B2, there aren't any more unique angles on the sawtooth compared to the W shaped trailing edge even though it has more 'sides'.
I was wondering if this is true in that the W shaped trailing edge has better stealth characteristics than the sawtooth edge and if so, is it significant?
The article mentions Bill Sweetman's Inside the Stealth Bomber book as its source.

The more "sides" a body has, the higher its radar reflection. Note that the MBB Lampyridae prototype had vastly lower (calculated, I must say) radar returns than both the Have Blue prototype and the F-117 that followed from it. When looking at the return of radar energy at grazing angles, it is the number of surfaces and edges that counts, not their size. The addition of the wing chord extension by Boeing to Northrop's original design made the B-2 possible, but increased its radar returns slightly over those of the original design.

Ah okay I see now, I thought it was the number of uniquely angled sides that mattered rather than the number of total sides. I would've thought that the waves would simply reflect along the same angles as the other parallel edges or is it more complex than that?

@Yourhelpismuchapprecciated Radar waves bounce off like light only when they hit a surface more pr less perpendicularly. At grazing angles they behave much more like waves of lower wavelengths which travel along the length of the material and produce diffuse emissions at edges and discontinuities.

thank you so much for your reply! That is really interesting, is there any good books or textbooks I can read to grasp this concept better?

@Yourhelpismuchapprecciated I learned that stuff from the people who designed the Lampy stealth research aircraft. At that time there were no books. I'm sure there are some now, I would go for basic physics first.

@Yourhelpismuchapprecciated Try this webpage which gives a pretty solid introduction into the topic. And here is part two.

Thank you so much for your help!

I think it has to do with the radar frequencies of interest that one is trying to defeat.  According to the article you referenced, adversaries are starting to deploy lower frequency (UHF) surveillance/tracking radars than existed when the B1 and B2 were designed.  The size of the "W's" on the trailing edge impact the stealthiness as a function of the radar's operating frequency.
Note that a lot of the details behind these designs is highly classified, held at the SAP/SAR level, and so only show up in very generic terms in publications.


text:
ERA runway close NOTAM
In the Flight Planning Stage
If enroute alternate airport runway close NOTAM isuue and its only the airport which in 60min circle on the flight route,
Should we find othere flight route or planning EDTO flight plan?
I think ERA is adequate airport. In case of an emergency, the ERA can be used regardless of the runway close NOTAM. So it not necessary to change flight route or EDTO.
What do you think about this ?

Is this a homework question? Because it looks like it.

Do you mean that you planned to use a single-runway airport as your alternate, but that during this flight a NOTAM was issued closing the runway, therefore leaving you without an alternate? Please [edit] your question to be more clear.

@GdD Or there may be machinery on the runway that makes landing there more dangerous than off-field.

If the only runway is closed, you should not (and cannot legally) use the airport as an alternate. If a usable runway is open at the airport, you can use the airport as an alternate.
Besides the rules, it may not be physically possible to use a closed runway because of equipment, partial or no pavement, water, etc., so it makes no sense to use it as an alternate.

An airport with no open runway is not an alternative as there's nowhere to land on it, so no it cannot be used for flight planning.
The runway may not exist if it is notamed closed, as it could be having pavement work going on. The taxiways may be clogged up with heavy equipment. A closed runway may mean a closed airport with no navaids, lights, emergency services, etc. If you landed there you may not even be able to leave the airport.
A closed airport along your route may be useful in an emergency, if you have no engine it's probably better than a field.


text:
(In English) is there a way/shorthand to refer to the minutes part of a plane's landing time? (I.e. omitting the hour)
For example, if a plane lands at 17:34, is it possible (in English) to only report the minutes (34) portion of the time? If so, how?
The reason/context for the question: I'm trying to translate something from another language (Mandarin); the speaker is a pilot, the addressee is an air traffic controller, but the conversation is taking place off work (so not precisely radio communications); the phrasing is "我XX分落地的," roughly translated to "I landed at xx minutes".

Why would you even do that?  If I had to do it for some reason, I'd write "HR+15", but I don't know if there is any kind of convention. If spoken, "15 past the hour".

I really think [edit]ing your question to indicate why and in what situation you would want to report only the minutes would be very helpful in having people understand the question and get you an appropriate answer. Seems to me that everyone is pretty confused.

@FreeMan Thank you for pointing this out. I've updated the question accordingly.

That helps a bit. It would probably help quite a bit more if you translated the rest of the Mandarin for us. There probably aren't too many here who speak it. Something like "Hey, Joe, I landed or 26L at 36  then exited on taxiway B" or whatever...

@FreeMan The Mandarin statement means something along the lines of "I landed at XX minutes," referring to the minutes part of the time when the plane landed.

There's no value in omitting the hour for brevity or any reason, and it sort of depends on who you are reporting to, the medium, and for what purpose.
You do save a few syllables saying "thirty four past" rather than "seventeen thirty four", but this may not always be the case depending on the number.
But yes, there is a way to say it clearly without mentioning the hour as long as you are still currently within that hour.  Whether you are talking to a dispatcher or ATC on the radio to close out a flight plan after landing at a non-towered airport you can simply say "thirty four past" (as mentioned above).  I've also heard things like "landed at time three four" and "on at three four".
As long as your meaning is clear there shouldn't be a problem, but I am not aware of any convention or phraseology (other than a company standard) specifically for omitting the hour.

I fly privately in Australia and whenever there is a requirement to communicate a time over the radio we say "at time two-zero", which indicates 20 minutes past the current hour. If it's not the current hour we revert to UTC and say "at time zero one two zero".
We don't have to report actual landing time to anyone. Times are generally required under IFR to tell ATC of the next waypoint ETA, or to tell traffic at untowered airfields of our ETA into the circuit pattern.
The reason we speak like this is the same reason we say "heading zero one three" instead of "heading thirteen". It is clearer over the radio to avoid misunderstandings.

It would simply be a convention for the particular report, that the landing (or Out or Off or In) time would be reported "here" using the two digits representing minutes past the hour. Those using that format would understand that "on at 36, in at 41" means XX:36 and XX:41 for the respective times.
There would have to be a way to ensure that the correct hour is eventually matched up with each time (i.e. for a case of a tarmac delay, landed at 0836 but blocked in at 0941), but there can be ways to do that.

Just in case this is in the wrong place, and you are looking for a linguistic answer, rather than a valid radio phraseology term...
You would say "34 minutes past the hour".
However, this is not necessarily correct radio phraseology that would be used in aviation.


text:
Can someone under training of MPL(A) or CPL(A) attain a PPL(A)? That is, is it possible to attain multiple licenses simultaneously?
As the title says, is this possible according to EASA?
According to the Swedish authorities, EASA do not allow attaining a second license before completing the first. But I have a hard time finding anything about it in any document.
Their reasoning is that while training for the second license it may interfere and cause confusion with the first one. I do not know how that is possible since you literally undergo the same training as PPL during the basics of MPL/CPL. Getting a PPL sounds more like a complementary thing than an interference.


text:
How is the power gained from exhaust of an engine calculated?
Ryan XV-5 Vertifan used exhaust gas from engine to rotate the fan inside the wing.
If I want to design an aircraft using this propulsion system how can I calculate how much power I can get from a specific engine exhaust?

This answer from space exploration stack exchange gives the formula for calculating power of an exhaust jet https://space.stackexchange.com/a/20075/6944 You need to know the thrust generated by the exhaust and the velocity of the exhaust.

We really answer this question in 2 parts.
The first is how much thrust the engine can generate at a given airspeed, either in pure thrust from exhaust gasses, or torque transferred to a mechanically rotating device, which in turn creates thrust.

this quality is known as specific impulse

Fortunately for aircraft such as the Ryan XV-5, the F35B, and helicopters, converting engine torque to vertical rotor lifting force, at lower airspeeds, can be far more efficient than simply exhausting gasses vertically.
Helicopters, such as the R-22,  can generate over 10 pounds of lifting force per horsepower.  Propeller driven aircraft get 3-4 lbs per horsepower flying horizontally, turbofans and turbojets less but at much higher speeds$^1$.
If you are considering a design such as the XV-5, power requirements per pound weight would be in the "ball park" of the F-35B.  With a larger a rotor over the top, like this one, they would be less.
Specific to the Ryan XV5 VTOL system, which was pneumaticly driven, lift could be theoretically calculated by the efficiency of the bleed air to drive the fans to an RPM sufficient to generate the amount of lift needed.
I must admit one aspect of the VTOL system struck me beyond theoretical calculations ... the lack of need for mechanical linkages.  The pneumatic drive was an ingenious application of the "turbocompound" energy recovery systems used in the huge piston engines of the day.
But again ... no mechanical linkages.  If it gets shot up, close the bleeds and land normally.
Furthermore ... could these fans be run hydraulicly, or even electricly?  Perhaps a design worth re-visiting.
$^1$ yes, jets do generate rotating mechanical energy, or "horsepower", to turn their turbines and fans.

So how do you calculate it?

My point is, if one cannot calculate thrust by following the instructions given in your answer, then you really haven't actually answered the question... have you?

@MichaelHall "calculating" the thrust of an experimental system is really a "chicken and egg" issue.  I favor testing, and deriving mathematical models from testing.  A highlighted section on "specific impulse" (for various propulsion systems) is there for your reading.  It should come as no surprise that (ducted) fans produce around 3x more thrust than "exhaust gasses" under static conditions.

To find the power you can get from the engine, we must calculate the energy of the exhaust gas and multiply it with the efficiency of the turbine. Note that this only gives the power output at the engine. To get the full answer, the efficiency of the transmission and of the fan in the wing need to be taken in to account too.
The power of the flow has two components, it's pressure and it's velocity. This is different from a rocket in vacuum or most general jet engines because in that case there is no pressure component. In vacuum there is no pressure, so the pressure component disappears. Most jet engines are designed to have an exit pressure similar the air pressure, so the pressure also drops out. Even if there is a difference, the energy from pressure is usually very small compared to the energy from the velocity. If we look at the flow in the engine before a turbine, the pressure is usually very high and the velocity low..
The general equation for thrust can be found here:
General Thrust Equation
A more detailed description of how to calculate the power can be found here:
Power Turbine Thermodynamics